Conventionally, a keyboard, mouse, touch panel, digitizer, tablet, pen type input section, and the like are used as the input section for entering information to a display device. Among them, the pen type input section, which is used to move displayed images, to select commands, or to perform similar operations on the display device, is a light-emitting pen, by which information is entered to a plate-shaped member that displays images associated to the images displayed on the display device.
Patent Document 1, for example, discloses a pen type input device that determines the coordinates of a point on a displayed image by detecting the distance that the tip of the pen moves and the direction of the movement. Specifically, Patent Document 1 discloses a pen type input device 2, which, as shown in FIG. 10, includes: a light source 104 that emits laser light to a writing surface 101; a photodiode 105 that detects the speckle pattern generated by the reflection against the writing surface 101; a moving amount calculation element that obtains the speed at which the pen tip moves based on the time correlation width of the detected speckle pattern, and calculates the moving amount by integrating the moving speed; a stress sensor 103 that detects stress in two directions that cross a pen axis 118 at a right angle; and a moving direction detecting element that detects the direction of the movement of the pen tip over the writing surface 101 based on the detected stress in two directions.